The present invention relates to bistable image display devices and more particularly to systems for addressing the row electrodes of the display device for duty cycle modulation.
Bistable display devices, such as matrices of gas discharge cells or electroluminescent devices, are well known within the display arts. A gas discharge display panel of this type is disclosed in U.S. Pat. No. 3,879,634 entitled "Manufacture and Operation of Gas Discharge Panel" issued on Apr. 22, 1975 to L. V. Pfaender et al. This panel comprises a plurality of individual image display cells arranged in an X-Y matrix. The individual cells are bistable in that they have only an on-off state without the capability of regulating the emitted light intensity. In the display of an image, the individual cells are sequentially addressed by an X-Y addressing scheme.
One of the difficulties encountered in adapting this type of display to television purposes is achieving gray scale or individual element brightness control. A conventional approach to achieving gray scale in bistable devices is to control the duty cycle of the individual cell. Specifically, the individual cell is turned on for a portion of the frame time of the image display which corresponds to the illumination level of the gray scale at that particular picture element. Sixty-four different brightness levels will provide sufficient brightness variation for television application. One method of achieving these various brightness levels for NTSC television is to divide the frame interval (33.4 Milliseconds) into six intervals corresponding to 8, 16, 32, 64, 128, and 256 horizontal line intervals. By proper selection of one or a combination of these six intervals, the 64 discrete brightness levels may be obtained. The necessary circuitry for controlling the various individual display cells to achieve gray scale using this technique has been fairly complex. The present invention seeks to simplify the addressing circuitry for achieving gray scale by duty cycle modulation.